1. Field of the Invention
The present invention relates to a magnetoresistance effect device, a method of manufacturing the same, a magnetic memory apparatus, a personal digital assistance, a magnetic reproducing head and a magnetic information reproducing apparatus.
2. Discussion of the Background
Solid magnetic memories including a Magnetic Random Access Memory (MRAM) using a MagnetoResistance (MR) effect element have been proposed. Further, a Tunnel MagnetoResistance effect device (TMR device) that uses a ferromagnetic tunnel junction has recently drawn attention as a component of a memory cell of the MRAM.
A conventional ferromagnetic tunnel junction has a three-layered film including a first ferromagnetic metal layer, a nonmagnetic dielectric layer and a second ferromagnetic metal layer. A current for sensing the tunnel junction's resistance flows from the first to the second of the ferromagnetic material layers via the nonmagnetic dielectric layer. The nonmagnetic dielectric layer forms a tunnel junction of the device, sometimes referred to as a tunnel dielectric layer. Each of the two ferromagnetic layers has a magnetization direction and a resistance value of the tunnel junction that changes in proportion to a cosine of a relative angle of magnetization directions of the first and second ferromagnetic layers.
The resistance value is at a minimum value when two magnetization directions of the ferromagnetic layers are parallel to each other and is a maximum value when the magnetization directions are not parallel to each other. A change in a resistance value (resistance amplitude) of the TMR device as much as 49.7% at room temperature (Appl. Phys. Lett. 77, 283 (2000)) has been reported.
The magnetization of one of the ferromagnetic layers of the ferromagnetic tunnel junction is fixed or pinned so the magnetization does not change or is inverted in an applied magnetic field, whereby the fixed magnetization ferromagnetic layer is used as a reference layer. The magnetization of the other ferromagnetic material layer is set to be free to rotate in the specific magnetic field, whereby the free magnetization ferromagnetic layer is used as a memory layer.
Several methods for fixing the magnetization of the reference layer and for allowing the magnetization free to rotate in the memory layer are disclosed in U.S. Pat. No. 5,159,513, the entire content of which is incorporated by reference.
In addition, the magnetization direction of the memory layer is changed in an applied magnetic field and the memory layer retains the changed magnetization direction. Further, the magnetization direction of the reference layer does not change in the applied magnetic field. Thus, the relative angle (i.e., parallel or antiparallel) of the magnetizations between the reference layer and the memory layer is changed. A binary value of “0” or “1” can be denoted to correspond to each of the parallel or antiparallel states.
Further, the magnetic information is written or recorded by inverting or changing the magnetization of the memory layer by the applied magnetic field. This is accomplished by generating a current to flow through a write line, which is electrically separate from but near the memory cell. The written or recorded information is read or reproduced by detecting a tunnel resistance change from a sense current flowing through the ferromagnetic tunnel junction. Further, a magnetic memory apparatus includes a number of the memory cells usually aligned in column and row directions on a same base, such as a semiconductor substrate.
A switching transistor may also be arranged with each memory cell and be coupled to the TMR device, similar to the Dynamic Random Access Memory (DRAM). Thus, an integrated peripheral circuit may select an arbitrary memory cell of the memory array. In addition, a diode can be used instead of the switching transistor and be placed at an intersection of a word line and a bit line (see U.S. Pat. Nos. 5,640,343 and 5,650,958, for example), where either the word line or the bit line is coupled to the TMR device via the diode, and the other of the word line or the bit line is directly coupled to the TMR device.
To form a highly integrated memory apparatus, a size of the memory cell including the TMR device should be reduced. However, as the size of ferromagnetic layer of the TMR device is reduced, a coercive force of the reduced ferromagnetic material layer becomes larger. A magnitude of a coercive force corresponds and is proportional to a magnitude of a switching magnetic field necessary for inverting a magnetization of the ferromagnetic memory layer. Therefore, an increase of the coercive force signifies an increase in the writing current, resulting in an increase in power consumption. An important subject to solve is to build the highly integrated magnetic memory apparatus with a reduction in coercive force of each ferromagnetic memory layer.
The TMR device usually has a rectangular plane shape, however, it is known that magnetic edge domains are produced in a small rectangular shape ferromagnetic layer (J. App. Phys. 81,5471 (1997)). The magnetic edge domain is formed, because magnetization at two shorter sides of the rectangular shape forms a pattern spirally rotated in line with the side to reduce a demagnetizing field energy.
For example, FIG. 1A is a plan view of an example of such a magnetic structure. As shown, the magnetic structure is an S-shaped magnetic domain structure, in which a magnetization at a center 11 is produced in a direction parallel to a magnetic anisotropy. Further, edge magnetic domains at both end portions 12 and 13 are produced and have a magnetization in a direction different from the center 11. FIG. 1B is a plan view of a C-shaped magnetic domain structure, in which the center 11 and both end portions 14 and 15 have different magnetization directions and in which the magnetizations of the end portions 14 and 15 are antiparallel to each other.
When the magnetization of the rectangular-shaped ferromagnetic memory layer starts to change or invert, each of the edge domains areas spreads. When the edge domains have magnetization directions antiparallel to each other as shown in FIG. 1B, a domain wall surrounds the center of the ferromagnetic material memory layer, whereby the coercive force of the memory layer is increased.
In addition, a ferromagnetic memory layer having an elliptic shape for reducing the edge domains is disclosed in U.S. Pat. No. 5,757,695. The elliptic-shaped ferromagnetic memory layer is formed to reduce a production of the edge domains and to promote a single magnetic domain in the entire layer, whereby a magnetization can be uniformly inverted over the entire ferromagnetic memory layer and an inversion magnetic field is reduced.
A ferromagnetic layer having a shape of a parallelogram has been proposed as a memory layer (see Japanese Patent Laid-Open No. H11-273337). In this case, although edge domains are present, the edge domains do not extend over a large area as in the case of a rectangular shape, and formation of very small domains during magnetization inversion is prevented. Therefore, magnetization of the memory layer can be inverted substantially uniformly. As a result, the inversion magnetic field can be reduced.
As a method of preventing a change of the complicated magnetic structure, magnetically fixing edge domains of the memory layer has been examined (see U.S. Pat. No. 5,748,524 and Japanese Patent Laid-Open No. 2000-100153).
Further, a tri-layered film including two ferromagnetic layers and a nonmagnetic layer interposed therebetween is also introduced as a memory layer for reducing an inversion magnetic field. The tri-layered film includes antiferromagnetic coupling between the two ferromagnetic layers and its magnetization as a whole is relatively lower than a single memory layer having a same shape (see Japanese Patent Laid-Open No. H9-25162, Japanese Patent Application No. H9-263741 and U.S. Pat. No. 5,953,248). The two ferromagnetic layers of the tri-layered film are different in magnetic moments or in their film thickness. Further, the magnetizations of two layers are in antiparallel directions by antiferromagnetic coupling, whereby larger portions of the two magnetizations are effectively cancelled and the tri-layered film as the memory layer is substantially equivalent to a single ferromagnetic layer having a small amount of magnetization in easy magnetization axis direction.
When a magnetic field in a direction inverse to the direction of the easy magnetization axis is provided to the tri-layered memory film, each magnetization of the respective ferromagnetic layers is inverted or changes while maintaining antiferromagnetic coupling. Magnetic force lines of the multi-layered film is closed in the tri-layered film and the influence of a demagnetizing field is inconsiderable, whereby a switching magnetic field of the memory film is determined by a difference of coercive forces between the two ferromagnetic layers, and the switching magnetic field for the multi-layered film is reduced.
Some methods for manufacturing the highly integrated MRAM having memory cells of a submicron size have been researched and method of using an electron beam (EB) drawing for making patterns of the memory cells is described in a journal, W. J. Gallagher et al., J. Appl. Phys. 81,3741 (1997).